1. Field of the Invention
The present invention relates to an image forming apparatus for exposing a photosensitive medium to light using a mask member, and more particularly to an image forming apparatus capable of maintaining and feeding the mask member to the correct position on the photosensitive medium in a time-expedient manner.
2. Description of Related Art
An image forming apparatus is known which uses, for example as in U.S. Pat. No. 4,810,614, light transmitting sheets (hereinafter referred to as mask members) having images thereon in black that are obtained by extracting red, green and blue components of a colored original image. With the image forming apparatus, photosensitive recording paper to which color capsules are applied, is sequentially exposed to filtered light using the three mask members to form an image.
An image forming apparatus which is shown in U.S. Pat. Nos. 4,884,082, 4,949,185 and 4,956,662 is operatively joined with a monochromatic printer. The monochromatic printer is a laser printer, in which a polygon scanner applies laser light over a precharged photosensitive drum to form an electrostatic latent image thereon. The photosensitive drum is then developed by a developing device whereupon the toner image on the photosensitive drum is transferred onto plain paper, or an overhead projector (OHP) sheet, supplied from a sheet cassette and is then fixed to the paper by an image fixing device. Usually, the laser printer records data transmitted from a non-illustrated host computer. In the meantime, the apparatus records a picture image by using three mask members. The laser printer produces the three mask members according to original color picture image information. When it is necessary to output a usual monochromatic picture image, a sheet of paper, on which a monochromatic picture image output from the laser printer is fixed, is fed to a discharge tray. When it is necessary to output a color picture image in the image forming apparatus, the three mask members, that is, one mask member for red, another mask member for green and a further mask member for blue are formed by a laser printer and are inserted into the image forming apparatus.
FIG. 9 is a sectional view showing a part of the route transmitted by a mask member in an image forming apparatus of the type which is shown in U.S. Pat. No. 4,949,185. A mask member 115R for red, another mask member 115G for green and a further mask member 115B for blue are formed by a laser printer or the like (not shown) and are inserted one-by-one into an inlet port 126 of the apparatus as shown in FIG. 9.
After the mask member 115R, for red, is inserted, it is transported downwardly to a left end of an endless belt 102 which extends in a horizontal direction. Endless belt 102 is formed from a transparent material and is electrically charged by a charging device (not shown). Thus, the mask member 115R is fixed to the endless belt 102 by force of static electricity and transported in the fixed condition by circulation of the endless belt 102. When the mask member 115R arrives at a predetermined exposure position, circulation of the endless belt 102 is stopped in a manner described below.
A sensor bar 105 supporting positioning mark sensors and mask member identifying mark sensors is positioned near one end of the feed belt 102. The positioning mark sensors serve to read positioning marks printed on predetermined portions of a mask member 115 for locating the mask member 115 in a given position, and the mask member identifying mark sensors serve to identify the color of the mask member 115 as described below.
FIG. 10 shows in detail the sensor bar 105 and the mask member 115. The mask member 115 includes a non-image area 115a bearing a mark Ml for indicating the position of the mask member 115 in the direction indicated by the arrow x, a mark M2 for indicating the position of the mask member 115 in the direction indicated by the arrow y, and marks M3, M4 for indicating the color of the mask member 115. The sensor bar 105 has sensors S1 thru S5. The sensors S1 and S2 are adapted for detecting the position of the mask member 115 in the direction indicated by the arrow x by sensing the marks Ml, M2, respectively. The sensor S3 is adapted for detecting the position of the mask member 115 in the direction indicated by the arrow y by sensing the mark M2. The sensors S4 and S5 are adapted for identifying the mask member 115 by sensing the marks M3, M4, respectively. Each of these sensors is in the form of a light-reflecting sensor comprising an LED and a photodiode, and senses the mark based on the difference between the reflectances of the mark and the background surface of the non-image area 115a.
The charging device electrostatically attracts the mask member 115R to the feed belt 102, and the feed belt 102 feeds the mask member 115 into an exposure position therein along the direction of the arrow x. Thereafter, the feed belt 102 is moved by actuators (not shown) in the directions indicated by the arrows y in FIG. 10 until the mask member 115R is properly positioned in the exposure position while the positioning marks Ml, M2 on the mask member 115R are being sensed by the positioning mark sensors S1, S2, S3 on the sensor bar 105.
A photosensitive recording medium 106 drawn out from a cartridge 104 extends below and parallel to the endless belt 102. A microcapsule paper, such as that disclosed in U.S. Pat. No. 4,399,209, may be adopted as the photosensitive recording medium 106. An exposure operation is performed while the photosensitive recording medium 106 and the mask member 115R are stopped opposed to one other at the exposure position. A lamp 108, serving as a light source for the exposure, is disposed above the endless belt 102 and three filters 110R, 110G and 110B for red, green and blue, respectively, are disposed below the lamp 108 such that they may selectively cover the lamp 108. Thus, the lamp 108 is first covered by the filter 110R for red. Lamp 108 and the filter 110R are integrally moved from a left end to a right end of the endless belt 102. Only the red component of the light from the lamp 108 is passed by the filter 110R and reaches the mask member 115R through the transparent endless belt 102. The red light component passes through the portion of the mask member 115R where there is no image or toner, that is at points that are not black, and is blocked by the black image portion of the mask member 115R. Consequently, the exposed surface of the photosensitive recording medium 106 is sensitized by red light in accordance with an image of the mask member 115R so that a latent image for red is formed thereon. After completion of such processing, the endless belt 102 is circulated to feed the mask member 115R to the right. The mask member 115R is fed out of the exposure position and transported upwardly along a route to the right of the apparatus as shown. The route branches into two routes at a position 114, the two routes leading to first and second buffers 116a and 116 b, respectively. Mask member 115R is fed into the upper route by a gate 118 provided at the branching position 114 and consequently is stored in the first buffer 116a.
Simultaneous with such storage of the mask member 115R, the mask member 115G, for green, is inserted into the image forming apparatus through the inlet port 126. The mask member 115G is then transported along the same route previously transmitted by the mask member 115R until it comes to the exposure position. It is to be noted that the photosensitive recording medium 106 is not transported during this operation. After the mask member 115G is positioned opposite to the photosensitive recording medium 106, the lamp 108 is moved from the left end to the right end of the endless belt 102 while covered by the filter 110G, thereby performing an exposure operation with only green light.
After exposure of the photosensitive recording medium 106 with green light, the mask member 115G is transported rightwardly by circulation of the endless belt 102 and fed at the branching position 114 into the lower route by the gate 118 so that it is stored in the second buffer 116b. Simultaneously, the mask member 115B, for blue, is inserted and transported to the exposure position. An exposure operation of the photosensitive recording medium 106 with blue light is performed in a manner similar to that previously performed with red and green light.
Thus, latent images for red, green and blue are formed in an overlapping relationship on the same surface of the photosensitive recording medium 106 by the multiple exposure of the three exposure operations. The exposed portion of the photosensitive recording medium 106 is then transported to a developing station while a new portion of the photosensitive recording medium 106 is drawn out from the cartridge 104 and introduced to the exposure position at which it is opposed to the endless belt 102.
Meanwhile, the mask member 115B is transported rightwardly and introduced into the first buffer 116a by the gate 118. Simultaneously, the mask member 115R, which has been stored in the first buffer 116a, is transported leftwardly to rejoin the route from the inlet port 126 side so that it is again transported to the exposure position, thereby reinitiating the exposure sequence.
In this manner, the mask members 115 are successively transported to the exposure position in a loop defined by the routes for mask member 115 movement provided in the apparatus. The gate 118 is controlled in accordance with the position of one of the buffers 116a and 116b in which a subsequently required mask member 115 is stored. After a predetermined number of images are formed in the manner described above, the mask members 115 are individually discharged by way of exit 120.
The endless belt 102 transports the mask member 115 in the Y-direction (FIG. 10) to register the mask member correctly. Immediately after the completion of exposure using the mask member 115, the endless belt 102 returns to its initial position together with the mask member 115 to receive the next mask member 115. The mask member 115 is moved toward the first buffer 116a or the second buffer 116b.
Each mask member 115 must be transported from the buffer 116a or the second buffer 116b to the exposure position and must be registered repeatedly with respect to the Y-direction to form a single image, thus requiring much time for registering the mask members 115.